Computer bios reading DIMM slot 1 as empty

[Adenov]

I have recently installled 2 more gbs of ram but in the BIOS it says slot 1 is unoccupied, whereas slot 2, 3 & 4 are occupied. I have switched all memory modules to that slot, and every time that slot fails to read. I know all the memory works because they work in other slots, just not this one. I've used a compressed air can and cleaned it, reseated the module many times and tried different dual channel combinations, but nothing works. Help!!

 

[GhislainG]

If you install a single module in that slot (and all other slots are empty), is it recognized? If not, then the slot is defective. If you inspect it with a magnifier glass, do you see something abnormal like a bent pin or dirst on a pin?